For decorative and holiday (e.g., Christmas) purposes it is frequently desirable to display a fresh cut or artificial evergreen tree in a natural upstanding posture. A portable tree stand is used to support the tree in this manner. Likewise, for marketing and retail selling purposes a stemmed merchandise rack is mounted in a display stand. The display stand may be motorized to rotate the Christmas tree or merchandise rack to draw attention and enhance aesthetic appeal.
When stemmed articles, such as Christmas trees and merchandise racks, are rotated in a motorized stand, it is foreseeable that an obstruction may occur and prevent the article from rotating. Both in the case of Christmas trees and merchandise display racks, it is also foreseeable that the obstruction may go unnoticed for a long period of time, during which the motor continues operating. In addition, both Christmas trees and merchandise racks in rotating display stands are susceptible to mischievous mishandling. For example, children may attempt to forcibly rotate the tree while the motor is operating, thus placing severe adverse loads on the motor. Furthermore, when a Christmas tree or merchandise rack in rotating display stand is not powered for rotation, i.e., when the rotating stand is turned off, forced rotation of the tree or rack can place harmful stresses on the motor, gear train, and other components in the tree stand.
The prior art teaches two possible solutions to these potentially damaging occurrences. One such solution is to size the motor so that it is capable of weathering long term obstructions and mischievous mishandling. This option, however, is generally disfavored due to the substantial increase in cost for a heavy duty motor. The second solution is to install a clutch between the motor and the rotating spindle. For example, U.S. Pat. No. 1,988,343 to Tacy, issued Mar. 29, 1932, and U.S. Pat. No. 2,587,788 also to Tacy, issued Mar. 4, 1952 both disclose simple friction clutch designs which permit slippage between the rotating spindle and the motor in the event of an obstruction. Such friction clutch designs represent a poor design choice in that the point of clutch disengagement cannot be established with certainty. While the clutch of one display stand may disengage at a 10 pound resistance, another identical display stand may not disengage until a 30 pound resistance is applied. Also, the break point cannot be maintained with certainty over long periods of time, and the clutch mechanism itself is prone to rapid attrition.
U.S. Pat. No. 3,042,350 to Lencioni, issued Jul. 3, 1962, discloses a cleat-type clutch where the bottom of the spindle is fitted with a series of female grooves and a driven gear is provided with a male cleat which seats in the female grooves. If an obstruction prevents the Christmas tree from rotating with the motor, the female grooves on the spindle will ride up and over the cleat. While this cleat-type design is an improvement over the earlier friction clutch design, it is still not possible to predict with certainty the point of clutch disengagement. In the Lencioni design, the weight of the Christmas tree provides the normal force to compress the female grooves over the male cleat and resist disengagement of the clutch. If the Christmas tree is particularly heavy, a greater resistance will be required to disengage the clutch. Conversely, if the Christmas tree is particularly light, it may not provide enough normal (compressive) force to overcome inertia at start-up.